1. Field of the Invention
The present invention generally concerns fluid dispensing systems, and particularly systems for mixing two fluids together in prescribed relative proportions and to a predetermined total volume. The present invention more particularly concerns post-mix soft drink dispensers for mixing together and dispensing carbonated water and flavored soft drink syrup in a prescribed mix ratio until a predetermined volume, typically one serving portion, has been dispensed.
2. Background of the Invention
Soft drinks are typically made by mixing a special syrup, produced by the drink manufacturers, with carbonated water. This mixing may transpire in canning and/or bottling facilities, or in post-mix soft drink dispensers at the point of sale. Post-mix soft drink dispensers typically inject the water and syrup simultaneously into a mixing chamber where they are mixed together. The mixed fluids are then dispensed through a nozzle into a drinking cup. The two fluids are normally supplied for coextensive time durations, and the mix ratio has typically been controlled using manually-adjustable metering pins.
To keep the quality of the mixed drinks at the desired level the syrup and water have to be mixed with a very high degree of accuracy. Syrup is desirably mixed with water in an exactly predetermined proportion, typically in the range from 1/7 to 1/3. Syrup manufacturers, and sellers of soft drinks, believe that the accuracy of the mixing, or "brix", should desirably be not worse than .+-.1%. Variations from the desired mix accuracy result in uneconomical use of the syrup component, and undesirable variations in the quality of the mixed drink product.
The desired mixing precision has not yet been achieved in post-mix soft drink dispensers. One source of error in mixing precision is due to variations in the absolute, and relative, pressures of the syrup and of the water during the period of dispensing operation, and between dispensing periods. These pressure changes tend to alter the flow rate of the syrup and/or the water, and of the syrup relative to the water. These changed flow rates in turn change the mixing, or "brix" of the beverage.
There are a number of previous soft drink dispensers that attempt to maintain a fixed mix ratio by manually adjusting a flow control metering pins in the syrup and water supply lines. The adjustment procedure is time consuming and cumbersome. A special container called a "standard measuring cup" is employed. The standard measuring cup has two reservoirs calibrated by a single scale, typically two vertical cylinders with a centrally-located scale. The volumes of the two reservoirs per scale unit of fill are in a fixed proportion, for example five to one (5:1). A soft drink dispenser is disassembled to a level which permits the syrup and water output flows to be separated. Each flow is plumbed into a corresponding reservoir of the standard measuring cup. The flows are adjusted to be in proper proportion in consideration of the observed filling of the standard measuring cup, normally by adjustment of the flow control metering pins. The procedure must be periodically reperformed in order to account for any long term changes in the pressures or viscosities of the dispensed fluid. Short term variations in flow rates during a single dispensing operation, or between individual dispensing operations, cannot be accounted for by periodic manual adjustments.
Soft drink dispensers that are automated for the adjustment of fluid flow rates exist. Many of these previous dispensers are complex. Most still require substantial manual adjustment when changing from one type of syrup to another. However, some previous flow-controlled dispensers are based on components as simple as a flow washer.
A flow washer is an annular ring, or washer, that is made of a flexible material. It is positioned in-line a bore flowing fluid. The flow washer typically presents a portion of a spheroidal surface having its convex side disposed upstream when the flow washer is positioned within the bore. Variations in fluid flow rate cause the flow washer to flex, thereby enlarging or constricting its central aperture.
Although much more expensive, and complex, flow regulation devices exist, flow washers are reasonably effective in precisely controlling flow rates over that range of pressures which are commonly experienced during beverage dispensing operations. If fluid pressure variations were the only source of corresponding variations in the relative proportions of dispensed water and syrup, then a mixing accuracy of .+-.1% could seemingly be achieved by existing flow-controlled beverage dispensing devices and systems.
Unfortunately, there is another, highly significant, factor contributing to variations in the accuracy of the mixing, or "brix", transpiring within post-mix soft drink dispensers. This factor is the change in the fluid friction, or fluid viscosity, of the mixed fluids. The syrup, in particular, has a viscosity that exhibits a very high dependance on temperature, and that undergoes significant changes during the normal environmental temperature variations that attend post-mix soft drink dispensing. When the fluid friction, or viscosity, of either fluid component changes then an equal pressure will cause a different amount of that fluid component to flow along the same path, thereby changing the relative proportions of the dispensed fluids. The carbonated water is generally less sensitive than the syrup to viscosity variations with temperature changes than is the water, thereby aggravating the problem of dispensing both syrup and water together in a precisely prescribed mix ratio over a range of temperatures, and over a corresponding range fluid viscosities.
Some soft drink dispensing systems attempt to measure fluid flow rates with flowmeters in order to adjust the flow rates, and to maintain them in a prescribed ratio. Unfortunately, the same fluid viscosity variations that cause problems with flow washer control of fluid flow also cause problems with flowmeters. Syrups can commonly vary in viscosity from .times.2 to .times.20 over the range between 32.degree. F. (0.degree. C.) and 194.degree. F. (90.degree. C.). These large variations cause commensurate variations in the signal outputs of previous viscosity-sensitive flowmeters. Previous systems dependent upon such viscosity-sensitive flowmeters for regulating fluid flow to dispense a (i) prescribed mix ratio and/or (ii) predetermined volume are correspondingly subject to undesirable error.
One previous system dealing with the full complexity of reliably providing accurate relative proportions of fluids despite variations in fluid pressure and/or fluid viscosity is shown in U.S. Pat. Ser. No. 4,487,333 for a FLUID DISPENSING SYSTEM. This fluid dispensing system employs syrup and water flowmeters in order to monitor the instantaneous flow rates of both the water and of the fluid. Responsively to this monitoring, separate syrup and water valves are controllably turned on and off, each independently at an appropriate duty cycle, in order to provide a prescribed mix ratio. The sensed, and controlled, fluid flow is directed to minimizing the effects of any pressure variations in the syrup and water supplies on the dispensing of the water and syrup in accurate and constant proportion.
However, the flowmeters of the previous fluid dispensing system are sensitive to changes in the viscosity of the fluid for which flow is sensed. The previous fluid dispensing system attempts to accommodate variations in the relative proportion of the water and syrup mix that are induced by changes in viscosity. In order to do so, a temperature sensor is used to sense the temperature of the syrup (that component of the mix that incurs the greatest changes in viscosity due to changes in temperature). A microprocessor-based electronics control circuit receives this sensed temperature, as well as the sensed flow rate. The control circuit references a separate, removable, personality module for each type of syrup. The module contains information on both the prescribed mix ratio for that particular syrup and the dependence of the syrup viscosity on changes in temperature. The control circuit uses this information to calculate the appropriate duty cycles that will control for pressure variations, and also for viscosity variations, in the dispensed syrup component.
The previous fluid dispensing system is obviously complex and expensive. It performs any one dispensing task on the basis of laboratory information contained within the removable personality module This information regards both the desired mix ratio and the viscosity characteristics of the syrup component. The personality module and its information contents must be customized for each individual type of syrup--an impossible task.
Moreover, the advanced fluid dispensing system of U.S. Pat. Ser. No. 4,487,333 attempts to effect fluid flow control by cycling a solenoid valve on and off at a variable duty cycle. The cycling induce noise and vibration. An induced hammering effect is detrimental to plumbing. A solenoid valve is typically a two-stated, on or off, device and is ill adapted for flow control.
In another matter, one known method determining the total amount of mixed fluids dispensed has been to enable fluids flows for a preset period of time. Because the total volume of mixed fluid dispensed is dependent upon the rates of the fluids' flows, the flow rates must be precisely controlled to be of a predetermined magnitude, or must at least be precisely known. Any variation in the predetermined flow, or measured flow, magnitude of either or both fluids results in an undesirable deviation in the volume of mixed fluids that is dispensed.
According to these limitations of previous fluid dispensing systems, it would be desirable that a fluid dispensing system, particularly for the post-mix dispensing of soft drinks, should operate to mix together and dispense two fluids at a prescribed mix ratio regardless of variations in the fluid pressure of either or both fluids, and regardless of variations in the viscosities of either or both fluids. The dispensing system would desirably function accurately with all syrups regardless of their differing viscosities and regardless of changes in viscosity with changes in temperature. The system would desirably maintain the predetermined mix ratio of both fluids continuously throughout the dispensing process in order that an excess of one fluid, or the other, would not have to be added to the dispensed drink at the last in order to establish the desired overall mix ratio.
It would be desirable that the mix ratio should be readily and easily specifiable--including at non-integer, or at slightly stronger or slightly weaker, ratios as the situation dictates--by the same personnel that otherwise and elsetimes use the post-mix dispenser to dispense soft drinks.
It would also be desirable if these unskilled personnel could readily and conveniently specify the volume of soft drink dispensed, including at volumes that do not represent an integer number of volume units and that may be, as the situation dictates, of a slightly greater or slightly lesser volume than is normally used to fill each one of a number of standard containers.
Finally, the fluid dispensing system accomplishing all this precise and readily conveniently specifiable control would desirably be inexpensive, reliable in operation, and essentially free of any requirements for alignment, adjustment, or scheduled maintenance.